1. Field of the Invention
The invention relates to a system for verifying the integrity of each one of a number of communication paths, illustratively telephone subscriber loops, that runs between a telephone company central office ("near end") location and a corresponding remote ("far end") location.
2. Description of the Prior Art
Frequently, a need arises in many applications for a highly reliable continuously operational communication path. Such applications include for example remote premise monitoring, data transmission and emergency communications. Because of the inherent reliability and widespread availability of telephone service, telephone lines, whether dedicated or dial-up, are being increasingly used to provide these communication paths.
One such application involves remote premise monitoring. Here, a remote premise, such as for example a residence, a business office or an unmanned site situated at a remote location, is outfitted with an electronic monitoring system that contains sensor(s) to detect the occurrence of one or more specific alarm conditions, e.g. fire, smoke, high temperature, intrusion, flooding or malfunction of a specific machine. These sensors are connected to monitoring circuitry that frequently contains an automatic telephone dialer. The dialer is connected through a subscriber loop (line) to a telephone company central office. At an occurrence of an alarm condition, the automatic dialer goes off-hook and through the subscriber loop dials the telephone number of an alarm company. After a telephone connection has been established between the central office and appropriate receiving equipment at an alarm company, the monitoring circuitry sends a message to the receiving equipment. This message contains the location of the monitored premises and the specific alarm condition that occurred thereat. An individual at the alarm company interprets the message and then dispatches appropriate assistance, if needed, to that location.
The reliability of a telephonic based remote premise monitoring system depends in good measure upon the integrity of a subscriber line (loop). For example, various systems known in the art that rely on the use of dial-up lines can be thwarted if a subscriber loop connecting the monitored premise with the telephone company central office is severed or if the loop breaks. Specifically, inasmuch as these prior art systems do not detect the occurrence of any interruption in the integrity of the subscriber loop, the receiving equipment typically remains unaware of it. Moreover, for reasons of cost and/or complexity, these prior art remote monitoring systems frequently do not contain back-up communication equipment, e.g. a radio link, that activates upon the failure of a primary communications link, e.g. a telephone line. As such, once the subscriber loop connection between the monitoring equipment and the telephone company central office is lost, the monitoring equipment becomes substantially unable to communicate the occurrence of an alarm condition to the alarm company. Hence, the remote premises become substantially unprotected.
Therefore, in telephonic based remote monitoring systems, a critical need exists to continuously monitor the integrity of subscriber loops that connect monitored premises to a telephone company central office and to provide notification of any loss of integrity.
One prior art remote alarm monitoring system that attempts to verify subscriber loop integrity is disclosed in U.S. Pat. No. 4,528,423 (issued to R. T. James et al on July 9, 1985 and hereinafter referred to as the '423 patent) and U.S. Pat. No. 4,442,320 (issued to R. T. James et al on Apr. 10, 1984 and hereinafter referred to as the '320 patent). Here, this system relies on a scanner preferably located at a telephone company central office that periodically interrogates the status of a number of remotely located subscriber terminal units through corresponding subscriber loops. Each subscriber terminal unit is located at a remote location and is connected to various sensors for monitoring certain alarm conditions thereat. If a non-alarm state exists at a location, the terminal unit situated thereat continuously transmits a corresponding sub-audible low frequency tone, approximately 25 Hz, over its corresponding subscriber loop. However, no such tone is generated if the terminal unit detects an alarm condition. Any interruption in the low frequency tone whether caused by an actual alarm condition detected by the terminal unit or a break in the subscriber loop is detected by the scanner as an alarm condition. In response to this occurrence, the scanner interrogates the terminal unit by emitting relatively high frequency FSK (frequency shift keyed) signals over the subscriber loop. The terminal unit replies again, if it can, using relatively high frequency FSK signaling, to provide the cause of the alarm. Thereafter, the scanner provides appropriate notification of any alarm condition so that proper corrective action can be undertaken.
Another prior art remote monitoring alarm system that attempts to verify subscriber loop integrity is disclosed in U.S. Pat. No. 3,930,246 (issued to S. V. Campbell on Dec. 30, 1975 and hereinafter referred to as the '246 patent). Here, circuitry located at a central station transmits a 3 kHz tone during a test interval over a telephone line to a remote station. Line integrity monitoring circuitry at a remote station receives this tone, divides it in half and transmits a resulting 1.5 kHz tone back over the telephone line to the central station. Upon receiving the 1.5 kHz tone, the central station ceases transmission of the 3 kHz tone until the next successive test cycle. If the 1.5 kHz tone is received within a prescribed period of time, which is sufficiently long to account for propagation delays and the like, then the telephone line is assumed to be operating satisfactorily. Alternatively, if the 1.5 kHz tone is not received, then the 3 kHz tone will remain on for longer than the prescribed period; in which case, the line is assumed to be broken and a suitable alarm is activated.
Unfortunately, these prior art systems possess a number of drawbacks that limit their utility. First, a subscriber loop can exist in two states: either off-hook and on-hook. The voltage on the loop changes dramatically between an on-hook condition, where the voltage and loop impedance are both relatively high, to an off-hook condition when the telephone is essentially shorting out the line and both the loop impedance and loop voltage are consequently relatively low. If the level of the tone is set to a particular level for an off-hook condition, as it frequently is, then whenever the line goes on-hook the level of the tone as it appears on the loop will increase substantially. This, in turn, frequently causes cross talk in other subscriber loops located in the telephone company central office. Alternatively, if the voltage of the tone is set to a particular level for an on-hook condition, then whenever the loop goes off-hook, the loop impedance decreases substantially thereby dramatically attenuating the level of the tone. As a result, the tone becomes quite difficult to detect at the telephone company central office. In addition, the presence of a very high level tone on the loop as it goes on-hook may imitate a telephone company signalling or transmission frequency and thereby disadvantageously cause equipment connected to a subscriber side of the loop to incorrectly assume that the loop is still off-hook when in fact it is not.
Second, use of single tones for subscriber loop verification purposes often does not provide sufficient security against tampering. In particular, a person, possessing some skill in electronics and interested in thwarting the remote monitoring system, could with rudimentary test equipment determine the frequency of the tone. Once the frequency is known, that person could duplicate the tone using a relatively simple signal generator. Having done this, the person could terminate the loop with the generator and then sever the remainder of the subscriber loop leading to the remote premise and its associated subscriber unit thereby effectively isolating the remote monitoring equipment from the telephone company central office and hence from the receiving equipment at the alarm company. This, in turn, disadvantageously negates the protection provided by the monitoring system to the remote premise. Furthermore, apart from the relative ease of single tone duplication, use of identical subscriber units also degrades security. Specifically, if every subscriber unit used a tone of the same frequency, then a person could readily terminate a subscriber loop with a separate subscriber unit and then sever the subscriber loop leading into the remote premise thereby effectively isolating the remote premise from the receiving equipment located at the alarm company. Since this disconnection would not be detected at the alarm company, its occurrence would likely negate any protection provided by the monitoring system to the remote premise.
Lastly, remote monitoring systems, particularly those shown in '423 and '320 patents, that have subscriber loop verification capabilities tend to be complex and rather expensive to implement.
Thus, a need exists in the art for a relatively simple and inexpensive subscriber loop verification system for verifying the integrity of each one of a number of subscriber loops that runs between a telephone company central office and a corresponding remote location. This system should provide a relatively high degree of security against tampering and, if tones are used, use a tone voltage level that does not cause cross talk onto other subscriber loops at the central office or imitate a telephone company signalling or transmission frequency to equipment connected to the subscriber side of a loop being verified whenever that loop goes on-hook. In addition, the tone should not be difficult to detect at the telephone company central office whenever the loop being verified goes off-hook.